An Envirogen Technologies Whitepaper

August 2011

1

Treatment of Selenium-Containing Coal Mining 

Wastewater with Fluidized Bed Reactor Technology

Proven Systems Provide Major Cost Advantages 

and “Best-in-Class” Performance

An Envirogen Technologies Whitepaper | August 2011
Treatment of Selenium-Containing Coal Mining Wastewaters 2

Introduction

In the coal mining industry today, selenium, in its many forms, is gaining 

attention. New regulations covering selenium discharge are either in 

place or being developed. It has been said that handling selenium 

wastes will increasingly become a limiting factor on the ability of the coal 

mining industry to grow. While a number of physical/chemical 

approaches show promise in selenium treatment, when considering 

optimal cost and treatment efficacy issues, biological treatment – particularly fixed-film biological 

treatment - is one of the most promising approaches to managing selenium-containing wastewaters. 

Envirogen Technologies offers a patented fixed-film fluidized bed reactor (FBR) biological treatment 

technology that has been proven as a „best in class‟ approach for handling medium to high flow rates of 

selenium-containing wastewater. Envirogen FBR technology has several distinct features that make it 

ideally suited for the coal mining industry. 

The noteworthy features of Envirogen FBR 

technology include its ability to consistently reduce 

selenium levels to less than 5 μg/L, shorter required 

residence times for treatment and a smaller overall 

footprint. Its flexibility in the choice of electron donor 

chemicals can translate into capital and operating 

cost savings with reduced solids generation. It also 

responds well to changes in feed flow and 

composition, consistently achieving discharge limit 

conditions. In addition, these systems can be 

modular, with all-weather protection where desired. These features, combined with decades of 

experience by Envirogen‟s team, allow the design and installation of FBR systems with capital costs that 

are a fraction of the cost of competitive fixed-film biological systems.

The selenium dilemma

Waste rock is generally the primary source of selenium in coal mining operations. When exposed to

water, various selenium species will leach or migrate from the rock and enter the environment. Selenium 

occurs in various valence states from -2 to +6. The speciation of selenium plays a critical role in the 

effectiveness of any approach for removal, especially to low levels. In aqueous environments, selenium

is most often found as the oxygenated anions of selenite, Se(IV) and selenate, Se(VI). Selenium in mine 

runoff most often occurs in the soluble form .Selenium is commonly found in mining wastewaters in 

concentrations ranging from 3 to >12,000 μg/L. The US National Primary Drinking Water Standard MCL is 

50 μg/L for selenium. The National Fresh Water Quality Standard is 5 μg/L for selenium. The U.S. Fish 
Treatment of Selenium-Containing Coal Mining Wastewaters 3

and Wildlife Service has recommended that the National Fresh Water Quality Standard be lowered to 2 

μg/L to protect fish, waterfowl, and endangered aquatic species. Several states have followed with 

enforcement actions at these same low levels. In Canada, permits may require stakeholders to monitor 

levels in water or biota, or to comply with Canadian Water Quality Guidelines of 1 μg/L in surface waters. 

Province and local regulations may vary from 

these requirements.

Treatment of selenium in coal mining wastewaters 

presents a series of challenges. It can be present 

in relatively dilute concentrations and in streams 

with variable and often high flow rates. It is 

present in many soluble and particulate forms, 

which may affect process design and treatability. 

The conditions of influent selenium-containing 

wastewaters can vary widely, with issues such as 

temperature, pH, total dissolved solids and other contaminants present, affecting the ability to selectively 

and economically remove selenium. Treatment of selenium often results in the generation of a 

concentrated by-product requiring disposal, and re-release from residuals can occur. Beyond these 

issues, the physical environment of coal mining operations offers challenges such as remoteness and 

harsh and/or cold weather. Currently available selenium treatment technology (Table 1) includes 

biological treatment and a number of physical-chemical processes – such as ion exchange, membrane 

filtration and adsorption – that vary widely in efficiency and cost. Most treatment options remain either too

costly and/or unproven for selenium-containing waste streams for reasons that include removal levels, 

variability in volume and composition of influents, system sizing and logistics considerations, as well as 

operating requirements. 

Table 1

Overview of Technologies for Removal of Selenium in Water

Physical Chemical Biological

Reverse osmosis Iron co-precipitation Algal-bacterial Se removal

Nanofiltration Zero valent iron Algal volatilization

Ion exchange Cupric co-precipitation Heterotrophic microbial reduction

Ferrihydrite adsorption Electrocoagulation Constructed wetlands

Activated alumina adsorption Enhanced cementation Phytoremediation

Photoreduction

Carbohudrazide
Treatment of Selenium-Containing Coal Mining Wastewaters 4

Biological Treatment of Selenium-Containing Wastewaters

Biological treatment of selenium-containing wastewaters has been an area of significant interest in the 

coal mining community recently. In general, biological treatment offers a low cost alternative to more 

expensive physical and chemical treatment methods and is effective in cold climates. Additionally, it has 

the proven ability to meet regulatory selenium limits.

Biological systems catalyze the reduction of selenium. Selenate can be reduced to selenite, and both 

selenate and selenite can be reduced to elemental selenium. 

SeO4

2-

+ organic carbon → SeO3

2-

+ organic carbon → Se0 + CO2 + H2O

These reactions are desirable because elemental selenium is virtually insoluble and therefore can be 

removed from the environment through conventional liquid/solids separation and disposal. In biological 

treatment, the conversion of selenium anions to elemental selenium is accomplished via biologically 

catalyzed reduction. Selenium-reducing bacteria (Figure 1) are considered heterotrophic. They utilize 

organic carbon as their electron donor and selenate/selenite as their electron acceptors. Commonly used 

electron donor materials include methanol, acetate, citric acid and molasses. New „designer‟ electron 

donors, generally composed of complex carbohydrates and organic process by-products that have been 

proven effective for selenium reduction and precipitation, are also available.

Figure 1: Selenium Reduction & Precipitation

One of the major issues in biological treatment of selenium is the presence of other anions in the influent 

streams. Both oxygen and nitrate are more favorable electron acceptors than oxidized forms of selenium. 

Figure 2 shows common anions and their order of biological reduction based on ORP. The order use of 

electron acceptors follows the more energetically favorable reactions first. Also, reduction enzyme 

systems appear to have evolved such that reduction enzymes are generally inhibited by electron 

acceptors that are more energetically favorable, particularly if the microorganism contains more than one 

reduction system.
Treatment of Selenium-Containing Coal Mining Wastewaters 5

Figure 2: Redox Profile of Biological Reduction

From a practical standpoint, these relationships are critical. The presence of either oxygen or nitrate will 

limit selenium reduction. Even in anoxic biological systems, the presence of nitrates is an important 

consideration in system design and the selection of microorganisms and electron donors. Many coal 

mining wastewaters contain high levels of nitrate that will be preferentially reduced before selenium, and

must be removed before effective selenium reduction will occur. Sufficient electron donor material is 

required to reduce both nitrate and selenium. The FBR process offers the potential benefit of 

denitrification and selenium reduction in a single step.

Active, fixed-film biological treatment systems have received significant attention from researchers and 

the coal mining community because of their potential to work with a broad range of selenium 

concentrations in wastewaters – including very dilute streams – and for their ability to reduce selenium to 

very low levels. These outcomes are due to the fact that the heterotrophic bacteria are retained in the 

bioreactor for relatively long periods of time, improving the chance that they will come in contact with the 

contaminants and the ability of these systems to control reaction conditions more precisely. Additionally, 

because of the tendency of the microbes to attach to the solid media and to form dense biomass films, 

these reactors can have high biomass concentrations. This is especially the case with the Envirogen FBR 

technology.
Treatment of Selenium-Containing Coal Mining Wastewaters 6

Today, there are two different types of active, fixed-film systems that have received significant field testing 

with selenium-containing coal mining wastewaters – the fixed-film downflow filter (also known as a 

„packed bed reactor‟ – PBR) and the fluidized bed reactor (FBR). In evaluating active, fixed-film bioreactor 

technology, the following issues are salient in choosing one of these approaches: 

 Treatment efficacy/efficiency 

 Hydraulic residence times

 System footprint

 Flexibility in handling variable & high 

flow rates

 Flexibility with electron donor material

 Ability to handle solids & metals

 Recovery from upsets

 Track record of technology

 Cost of capital

 Cost of operation

 Ease of operation

The remainder of this paper will present information related to Envirogen Technologies‟ fluidized bed 

reactor technology – and where applicable, compare and contrast technical and operating features with 

packed bed reactor technology.

Envirogen Fluidized Bed Reactor Technology

Proven technology with critical advantages for the coal industry

Envirogen‟s FBR is an active, fixed-film bioreactor that fosters the growth 

of microorganisms on a hydraulically fluidized bed of fine media. In this 

type of reactor, a fluid is passed through a granular solid material at 

velocities sufficient to suspend or fluidize the solid media.

In Envirogen FBR systems, media types include sand and activated 

carbon media that are manufactured to exacting specifications for 

hardness, shape, size, uniformity, density and impurity levels. The small 

fluidized media in the FBR provide an extremely large active surface area 

upon which microorganisms can grow while treating contaminants. A 

large biomass inventory is produced while maintaining thin films, reducing 

mass transfer limitations and offering high volumetric efficiency.

Envirogen manufactures both aerobic and anoxic FBR systems. Aerobic systems utilize air or oxygen for 

removal of organics. Anoxic FBR systems are the technology of choice for inorganic contaminants such 

as perchlorate, nitrate and selenium. Other distinguishing features of Envirogen FBRs include patented 

biomass control systems that are key to retaining media and steady-state operation and plug flow; custom 

molded parts; proprietary design of internal vessel components and proprietary and patented controls for 

chemical feed systems.
Treatment of Selenium-Containing Coal Mining Wastewaters 7

Base System Design

Envirogen FBR installations typically feature one or more vessels in single- or two-stage configurations, 

depending on influent water characteristics and discharge limits (Figure 3). A range of materials can be 

used for vessel construction, including stainless steel, fiberglass, lined-carbon steel or concrete. 

Prefabricated FBR vessels from 2- to 14-feet diameter and up to 30-feet in height are available – offering 

very deep beds in comparison to other fixed-film technology. This vertical orientation is one of the factors 

that contribute to the FBR‟s smaller footprint compared to packed bed reactors or other biological 

treatment systems. Field-fabricated concrete or metal vessels are also available for very large 

applications. Overall system packages are outfitted with fluidization pump(s), piping, valves, chemical 

feed pumps and controls that may be field assembled or pre-assembled based on client preferences. 

Most Envirogen FBR systems are equipped with programmable logic control and may be provided with a 

SCADA package, telemetry and motor controls options.

Figure 3: Envirogen FBR for Perchlorate Removal

System Operation

During start-up, the FBR is seeded with heterotrophic bacteria that are suited for nitrate and selenium 

removal (Figure 4). Electron donor materials and nutrients are pumped into the FBR to promote microbial 

growth. 
Treatment of Selenium-Containing Coal Mining Wastewaters 8

Figure 4: Start-up of FBR system

Selenium-containing wastewater is pumped from a feed tank into the FBR in an upflow direction to 

suspend the media. As microorganisms grow on the media, the fluidized bed height expands (Figure 5). 

Figure 5: Fluidization of FBR media
Treatment of Selenium-Containing Coal Mining Wastewaters 9

With time, a biofilm develops on the media surface. Nitrate and selenium reduction occur on this biofilm. 

Treated water from the FBR system is discharged to a downstream liquid/solids separation system where 

the biological solids and elemental selenium are separated. Thickened or dewatered bio-solids and 

elemental selenium are disposed.

One of the key features of operation is the 

technology used to fluidize the media bed 

and to uniformly distribute fluidization flow to 

the system. This is accomplished with the 

design and placement of the distribution 

nozzles and the sizing and arrangement of 

the header-lateral distributors. Uniformity of 

flow is critical to establishing steady-state 

plug flow conditions and the associated 

optimized performance.

Another key feature of these FBR systems 

is Envirogen‟s proprietary technology for biomass control. In all properly functioning biological systems 

some of the energy that is derived from the consumption of electron donor is used by the microbes for 

biomass growth. This results in the production of excess biomass or “yield” that must be removed from 

the bioreactor. In packed bed reactors this is accomplished with periodic backwash operations that 

remove large quantities of biomass using very high backwash rates – creating turbulence and shear to 

separate and wash out accumulated biomass. 

In Envirogen‟s FBR systems biomass is removed in a steady-state manner with Envirogen‟s patented 

biomass control devices. These systems allow the FBR to operate with an optimized and consistently

high biomass concentration, resulting in reliably high performance. 

Limitations inherent in packed bed reactors render them incapable of handling high feed nitrate, selenium 

and suspended solids concentrations. This is due to problems of rapid plugging, nitrogen gas binding,

channeling and high backwash requirements. With Envirogen‟s patented biomass controls, FBRs 

effectively handle feed loads that are greater than an order of magnitude higher then packed bed 

reactors.

Design Flexibility 

The design flexibility available through utilization of the FBR with respect to vessel heights and 

configuration, materials of construction, modularity, Envirogen‟s patented bed height control, biomass 
Treatment of Selenium-Containing Coal Mining Wastewaters 10

control, proprietary chemical feed configurations and flexibility in electron donor selection all combine to 

produce improved performance, reduced capital and operating costs, and minimized excess biomass 

production and associated processing and disposal costs. Furthermore, this allows for the use of 

alternate electron donors based on considerations of availability, cost and performance.

Advantages of the Envirogen FBR technology for the coal mining industry

Extensive testing of the Envirogen FBR technology has shown that it consistently offers highly efficient 

removal of selenium from coal mining wastewaters at dramatically lower cost levels than competitive 

technologies. Three aspects of Envirogen FBR systems support this success:

 The high performance nature of FBR treatment technology

 Proprietary & patented features of Envirogen‟s FBR systems

 Envirogen‟s decades of experience with FBR in treating inorganic contaminants

The following issues highlight the Envirogen FBR systems‟ performance advantages for the coal mining 

industry:

Optimization of microorganism efficiency

One of the key advantages of FBR technology over packed bed reactors is the lack of „channeling‟ that 

occurs in FBR systems. Packed bed reactors tend to develop preferential pathways for flow due to solids 

build-up in the bed and nitrogen gas binding caused by conversion of nitrate to nitrogen gas. This can 

cause pressure drop in the typical fixed-film system as well as short-circuiting or partial flow by-passing. 

These deviations from steady state plug flow conditions result in reduced performance. In the fluidized 

bed reactor, because of the even distribution of flow, a very high fraction of the biomass within the 

bioreactor is utilized – meaning increased treatment efficiency. These issues directly affect the hydraulic 

residence time (HRT) required and explain why the footprints for Envirogen FBR systems are so much 

smaller than competitive packed bed bioreactors for the treatment of nitrate and selenium.

Steady-state operation

Envirogen FBR systems are designed to be operated continuously – meaning they do not require cyclical 

backwash operations. Packed bed reactors require periodic backwashing to slough off excess microbial 

growth, to eliminate channeling and to remove nitrogen gas pockets that have built up during the running 

part of the packed bed reactor operating cycle. Frequency of backwashing is related to the loading of 

contaminants in the influent stream and requires large pumps and intermittent operation. In addition to 

increased energy costs, backwashing limits the flow of wastewater through the entire system since it is 

necessary to remove the backwashed unit from service, recover, clarify and recycle the dirty backwash 

suspension. Recycled backwash water adds to the feed loading and thereby limits net throughput. 
Treatment of Selenium-Containing Coal Mining Wastewaters 11

Perhaps more importantly, the biomass in the system requires time to regrow following a backwash,

further increasing the required HRT. With higher loadings of feed nitrate, selenium and/or suspended 

solids, the backwashing requirements can become so excessive as to render the packed bed bioreactor 

ineffective or completely impractical.

Improved overall water quality

Besides selenium and nitrate, another parameter that may be of concern is soluble biological oxygen 

demand (BOD) or chemical oxygen demand (COD). Mine run-off waters are generally very low in BOD 

and COD. However, discharge from biological systems treating for these contaminants can contain 

excess un-reacted electron donor, resulting in soluble BOD or COD in the bioreactor effluent. This will 

depend on the electron donor used and the quality of the biological contacting system, both of which 

affect the completeness of the reactions. 

For example, some grades of molasses contain a substantial fraction of non-biodegradable or difficult to 

degrade compounds that will exit the bioreactor as soluble BOD and COD. Allowing for flexibility in the 

choice of the electron donor(s) to be used, especially as this relates to two-stage operation, can allow the 

system operator to control treated water BOD and COD. Additionally, the plug flow characteristics of the 

FBR tend to result in more complete bio-reactions and lower treated water concentrations.

Smaller overall footprint 

FBR systems have a smaller site footprint than other biological treatment systems. The key contributors 

to this feature are the vertical orientation of the FBR vessels and the efficiency of treatment (lower HRTs). 

Treatment efficacy is affected by a high concentration of biomass and the tall beds. This smaller footprint 

has a dramatic effect on construction and installation costs and schedules.

Dramatically lower hydraulic residence times

Despite having a smaller overall footprint, field studies have shown that FBR systems can achieve 

required performance with dramatically shorter HRTs than packed bed systems. This is due primarily to 

deep beds and steady-state plug flow operation. HRTs for typical FBR systems used in selenium 

treatment for coal mining operations are 1/3 to 1/10 those required for packed bed reactors.

Electron donor options

Envirogen offers the ability to customize electron donor materials based on decades of experience with 

the FBR technology. Key issues that are considered include treatment efficacy, overall water quality, 

operational ease of use, availability and cost. Table 3 shows a typical comparison of electron donor 

usage, costs and treated water quality as a function of alternate electron donors selection in a two-stage 

FBR system treating selenium and nitrate. 
Treatment of Selenium-Containing Coal Mining Wastewaters 12

Table 3: Electron Donors Comparative Evaluation

Molasses MicroCg 1

st stage Methanol

2

nd stage MicroCg

1

st stage Methanol

2

nd stage Molasses

Usage L/day 4260 3135

1362 (1st stage)

627 (2nd stage)

1362 (1st stage)

852 (2nd stage)

US$/L delivered 0.373 0.528

0.396 (methanol)

0.528 (molasses)

0.396 (methanol)

0.373 (molasses)

US$/yr 580,000 604,000 317,700 312,900

FBR Effluent TSS mg/L 30 25 22.6 23.6

FBR Effluent filtered COD 

mg/L 100 50 20 30

Robustness of the system

Envirogen FBR systems have a proven history over the past 30 years, operating effectively over a wide 

range of feed flow rates and influent types. The systems are tolerant of high feed concentrations of total 

suspended solids (TSS) and metals. In addition, these systems have the ability to recover quickly from 

upsets such as power outages and loss of chemical feeds. 

Modularity & extreme environment performance

In many cases, Envirogen FBR systems can be pre-fabricated for delivery to a coal mining treatment site. 

This contrasts with packed bed reactor systems which, because of their large size, are usually built-inplace. FBR systems may be supplied as prefabricated modules and associated equipment can be 

containerized in insulated and heated containers, per client requirements. This allows for ease of 

transportation and rapid installation – even in bad weather conditions.

Generally speaking, it is expected that biological treatment systems suffer loss of efficiency when used in 

cold weather environments (> 15o C). Recent tests by Envirogen on selenium-containing wastewater at a 

coal mining site showed that the FBR system performed effectively at influent water temperatures down to 

4o C.

Lower costs

One of the major advantages of employing an FBR in a selenium treatment application is a considerable

cost advantage in system design and installation compared with other commercially available 

technologies. This is primarily due to smaller system footprints as well as the ability to manage issues 

such as suspended solids and waste disposal in a much more effective fashion, over a broad range of 

influent flow rates. A recent third-party analysis performed for the North American Mining Council showed 

that initial capital costs for an FBR system can be 1/3 or less the cost of a packed bed reactor system 

designed for similar treatment requirements. 
Treatment of Selenium-Containing Coal Mining Wastewaters 13

Technology track record

Envirogen is an industry leader in the use of bioreactor technology, with over 150 systems installed in the 

United States. Envirogen FBRs have been used in similar reductive applications for more than 10 years in

applications such as perchlorate and nitrate, where low levels (<4 ppb) are consistently achieved at high 

flows (6,000 gpm) and in high TDS streams. The same anoxic operating regime is used for selenium

installations.

Recently, Envirogen FBR technology demonstrated the ability to achieve <5 ppb selenium over a 10-

month period in treating mining leachate at a U.S.-based coal mining site. Competitive trials run at the 

same site indicated that Envirogen FBR systems were the „technology of choice‟ for reliability and costeffectiveness. Summary results for this trial are shown in Table 4.

Table 4: Operating results from 10-month coal mining trial (USA)

A four-month trial was also run at a Canadian coal mine – examining Envirogen FBR technology in the 

presence of high nitrate levels and at colder influent water temperatures. Table 5 shows results of this 

trial.
Treatment of Selenium-Containing Coal Mining Wastewaters 14

Table 5: Operating results from 4-month coal mining trial (Canada)

Envirogen Lifecycle Cost Assurance Programs

In any environmental installation the most important – and often the most expensive – contributors to a

treatment program are the performance and operating costs over its 10- to 20-year life. Developed to

support all FBR installations, Envirogen Lifecycle Cost Assurance Programs are designed to ensure

performance, to deliver low lifecycle costs and to guarantee the life of the assets. Elements of an

Envirogen Lifecycle Cost Assurance Program range from basic O&M support services or the assignment

of Envirogen supervisory and management personnel to comprehensive operations programs wherein

Envirogen becomes a single line item on the operating budget for the system in question. They can even

include financing options for system design, construction and installation. Envirogen Lifecycle Cost 

Assurance Programs are typically 10 to 20 years or more, with the length generally following the 

anticipated life of the system in question. Guarantees are provided for system performance, operating 

cost components, asset life and overall lifecycle cost. In addition, Envirogen can also provide guarantees 

for cost elements such as utility consumption and chemical usage for the life of the contract. Through the 

resources and experience that Envirogen Technologies brings to design, fabrication, installation, and 

long-term operation, significant value can be realized via an Envirogen Lifecycle Cost Assurance 

Program. These include:

 Predictable and guaranteed costs for the lifetime of the asset

 Guaranteed performance, repair and maintenance costs, and regulatory compliance

in 2 stages
Treatment of Selenium-Containing Coal Mining Wastewaters 15

 Opportunities for cost reduction through process improvements, capital improvement and 

energy/chemical savings

 Risk protection via performance and cost guarantees

 Improved budget management (avoid unbudgeted cost excursions)

 Cost-effective access to industry specialists

 Continual process improvements

 Better utilization of capital

 A comprehensive EH&S Program

Conclusions

With selenium regulations having an increasing presence in the coal mining industry landscape, efficient, 

cost-effective treatment is critical to the growth of the industry. Envirogen FBR technology offers the 

industry‟s best, most broadly applicable solution to selenium treatment in systems that are field-tested 

and ready for design and deployment. The design of Envirogen‟s FBR systems ensures that selenium 

removal targets can be consistently and reliably met – at costs substantially below other commercially

available competitive technology. The experience of the Envirogen team with FBR technology for 

decades ensures that the goals for a cost-effective selenium treatment program can be met today and in 

the future.